The need for cleaner burning fuels has resulted in a continuing world wide effort to reduce sulfur levels in gasoline and diesel fuels. The reducing of gasoline and diesel sulfur is considered to be a means for improving air quality because of the negative impact the fuel sulfur has on the performance of automotive catalytic converters. The presence of oxides of sulfur in automotive engine exhaust inhibits and may irreversibly poison noble metal catalysts in the converter. Emissions from an inefficient or poisoned converter contain levels of non-combusted, non-methane hydrocarbon and oxides of nitrogen and carbon monoxide. Such emissions are catalyzed by sunlight to form ground level ozone, more commonly referred to as smog.
Most of the sulfur in gasoline comes from the thermally processed gasolines. Thermal processed gasolines such, as for example, thermally cracked gasoline, visbreaker gasoline, coker gasoline and catalytically cracked gasoline (hereinafter collectively called "cracked-gasoline") contains in part olefins, aromatics, and sulfur-containing compounds.
Since most gasolines, such as for example automobile gasolines, racing gasolines, aviation gasoline and boat gasolines contain a blend of at least in part cracked-gasoline, reduction of sulfur in cracked-gasoline will inherently serve to reduce the sulfur levels in such gasolines.
The public discussion about gasoline sulfur has not centered on whether or not sulfur levels should be reduced. A consensus has emerged that lower sulfur gasoline reduces automotive emissions and improves air quality. Thus the real debate has focused on the required level of reduction, the geographical areas in need of lower sulfur gasoline and the time frame for implementation.
As the concern over the impact of automotive air pollution continues, it is clear that further efforts to reduce the sulfur levels in automotive fuels will be required. While the current gasoline products contain about 330 part per million with continued efforts by the Environmental Protection Agency to secure reduced levels, it has been estimated that gasoline will have to have less than 50 parts per million of sulfur by the year 2010. (See Rock, K. L., Putman H. M., Improvements in FCC Gasoline Desulfurization via "Catalytic Distillation" presented at the 1998 National Petroleum Refiners Association Annual Meeting (AM-98-37)).
In addition to the need to be able to produce low sulfur content automotive fuels, there is also a need for a process which will have a minimal effect on the olefin content of such fuels so as to maintain the octane number (both research and motor octane number). Such a process would be desirable since saturation of olefins greatly affects the octane number. Such adverse effect on olefin content is generally due to the severe conditions normally employed to remove thiophenic compounds such as, for example, thiophenes and alkyl dibenzothiophenes, which are some of the most difficult sulfur-containing compounds to be removed from cracked-gasolines. In addition, there is a need to avoid a system wherein the conditions are such that the aromatic content of the cracked-gasoline is also lost through saturation.
Thus there is a need for a process wherein desulfurization is achieved and olefin retention is improved.
While it has been shown in my copending applications Desulfurization and Novel Sorbents for Same, Ser. No. 09/431,454, filed Nov. 1, 1999, and Ser. No. 09/431,370, filed Nov. 1, 1999, that a suitable system for the desulfurization of cracked-gasolines or diesel fuels is that employing a sorbent comprised of a reduced valence nickel or reduced valence cobalt metal on a zinc oxide, silica, alumina support, there is a continuous effort to develop additional systems which permit the desired desulfurization of such cracked-gasolines or diesel fuels and which will provide for alternative or improved desulfurization conditions to permit variations within the operation of the process.
It is thus an object of the present invention to provide a novel sorbent system for the removal of sulfur from fluid streams of cracked-gasoline.
Another object of this invention is to provide a process for the production of novel sorbents which are useful in the desulfurization of cracked-gasoline.
A further object of this invention is to provide a process for the removal of sulfur-containing compounds from cracked gasoline which minimizes the loss of olefin values in the resulting product.
A still further object of this invention is to provide a desulfurized cracked-gasoline that contains less than about 100 parts per million of sulfur based on the weight of the desulfurized cracked-gasoline and which provides for enhanced olefin retention in the cracked-gasoline.
Other aspects, objects and the several advantages of this invention will be apparent from the following description of the invention and the appended claims.
The present invention is based upon my discovery that through the utilization of a bimetallic promotor derived from a metal, metal oxide or metal oxide precursor wherein the metals are selected from the group consisting of cobalt, nickel, iron, manganese, copper, zinc, molybdenum, tungsten, silver, tin, vanadium and antimony and mixtures thereof and wherein the metals of such bimetallic promoter are in a substantially reduced valence state, preferably zero, and wherein such bimetallic promotor composite is supported on a suitable support there is achieved a novel sorbent composition which permits the ready removal of sulfur from cracked-gasoline while providing an unexpected improvement in olefin content retention in the resulting cracked-gasoline.
Accordingly, in one aspect of the present invention there is provided a novel sorbent suitable for the desulfurization of cracked-gasolines which is comprised of a bimetallic promotor on a suitable support wherein the valence of each of the metals of the bimetallic promotor have been substantially reduced and such reduced metals are present in an amount to permit the removal of sulfur from a cracked-gasoline while providing for enhanced olefin retention in the treated cracked-gasoline stream.
In accordance with another aspect of the present invention, there is provided a process for the preparation of novel sorbent compositions which comprises forming a suitable support in which the bimetallic promotor is well dispersed. Regardless of the manner in which the components of the novel sorbents of this invention are combined, the preparation comprises forming a wet mix, dough, paste or slurry thereof, particulating the wet mix, dough, paste or slurry thereof to form a particulate granule, extrudate, tablet, sphere, pellet, or microsphere thereof, drying the resulting solid particulate and calcining the dried particulate. The resulting calcined bimetallic promotor containing composition is then reduced with a suitable reducing agent, such as hydrogen, so as to produce a sorbent composition wherein the valence of the metals of the bimetallic promotor is substantially reduced and wherein the bimetallic promotor is present in an amount which is sufficient to permit the removal with same of sulfur from a cracked-gasoline.
In accordance with a further aspect of the present invention there is provided a process for the desulfurization of a cracked-gasoline which comprises desulfurizing in a desulfurization zone a cracked-gasoline with a bimetallic promotor containing sorbent, separating the desulfurized cracked-gasoline from the resulting sulfurized sorbent; regenerating at least a portion of the sulfurized sorbent so as to produce a regenerated desulfurized sorbent; activating at least a portion of the regenerated desulfurized sorbent to produce a bimetallic sorbent having a reduced metal valence content; and thereafter returning at least a portion of the resulting bimetallic promotor containing sorbent to the desulfurization zone.